Thin film solar cells typified by amorphous silicon solar cells and compound thin film solar cells allow for significant reduction in material costs and production costs as compared with conventional crystalline silicon solar cells. Therefore, research and development have been rapidly conducted on these thin film solar cells in recent years. Among these thin film solar cells, a CIGS solar cell which is a type of compound thin film solar cell produced by employing Group I, III and VI elements as constituents and including a light absorbing layer composed of an alloy of copper (Cu), indium (In), gallium (Ga) and selenium (Se) is particularly attractive, because the CIGS solar cell is excellent in solar light conversion efficiency (hereinafter referred to simply as “conversion efficiency”) and is produced without the use of silicon.
The light absorbing layer of the CIGS solar cell can be produced by a selenization process, a non-vacuum (nano-particle) process, a vacuum evaporation process or the like. The vacuum evaporation process is a film formation process in which a film is formed through evaporation by heating separate evaporation sources of Cu, In, Ga and Se and, therefore, is advantageous in that the feed amounts of the respective elements can be controlled for the formation of the film, making it possible to control the composition of the film in thickness direction.
A so-called three-stage process, which is a type of multi-source evaporation process out of the vacuum evaporation process, ensures the highest conversion efficiency. As shown in FIG. 8B, this process is divided into three stages. At the first stage, In, Ga and Se are deposited on a substrate through evaporation to form an (In,Ga)2Se3 film. At the second stage, the temperature of the substrate is elevated to 550° C., and Cu and Se are further deposited on the resulting substrate through evaporation to form a CIGS film having a Cu-rich composition. At this stage, two phases, i.e., a liquid phase Cu(2-x)Se and a solid phase CIGS, coexist in the CIGS film, and crystal grains rapidly grow to greater size in the presence of Cu(2-x)Se.
On the other hand, it is known that Cu(2-x)Se adversely affects the characteristic properties of the solar cell because of its lower electrical resistance. In the three-stage process, therefore, In, Ga and Se are deposited through evaporation at the third stage to reduce the amount of Cu(2-x)Se, so that the CIGS film has a composition slightly rich in Group-III element as a whole. The CIGS thin film obtained by the three-stage process has greater crystal grain diameters and, in addition, has a thin film crystalline structure having a crystallographically higher quality than a CIGS film formed by a conventional evaporation process (for example, PLT1).
Where the CIGS film formed by the three-stage process is used for a solar cell of a smaller-area device, the solar cell is indeed advantageous with a higher conversion efficiency. However, Cu(2-x)Se, which is a major component causing the crystal growth, is supplied in a liquid phase from the beginning, so that Cu is not necessarily uniformly diffused in the CIGS film and the crystal grains are not necessarily uniform in a strict sense. Where a large-area device is produced by utilizing the CIGS film, there is device-to-device variation in conversion efficiency with poorer reproducibility. Further, Cu(2-x)Se is supplied in a liquid phase and, therefore, is excessively taken into the film. Problematically, this reduces the characteristic properties of the device.